1. Field of the Invention. This invention is directed to mass flow measuring systems, in general, and to a system wherein a vastly improved operation is achieved with a novel insulator material and construction.
2. Prior Art. In conventional mass flow measuring systems, heat is applied to (1) a sensing tube which is used to conduct a fluid to be measured or (2) directly to a fluid. The temperature of the fluid is measured before and after the heat is applied. When the upstream temperature is equal to the unheated stream temperature, mass flow can be measured as inversely proportional to the temperature difference for a constant heat addition by means of a conventional bridge circuit. Reference is herewith made to U.S. Pat. No. 3,938,384 Mass Flow Meter with Reduced Attitude Sensitivity. R. F. Blair, assigned to Tylan Corporation. The cited patent provides a suitable detailed description of conventional mass flow measuring systems.
In particular, the referenced patent describes a dual coil arrangement which is used in measuring the mass flow of a fluid over a wide temperature range with reduced attitude sensitivity. The patent specifies the use of open cell foam material to encase the sensing tube coils of the measuring system. The result alleged by the prior patent is a minimization of natural convection around the sensor coils and, thereby, the reduction in attitude sensitivity from greater than 50% without the insulator, to less than 0.5% with the insulator. However, the "open cell foam" or an "open cell beaded foam" material has several distinct disadvantages in operation such as uneven temperature control, thermal degradation and the like. Consequently, an improved thermal control arrangement is highly desirable relative to the sensing tube portion of the measuring system.